Why did Lord Shiva drink the halahal poison alone when there were prominent gods present there who could have also done it?

 As per Srimad Bhagavatam (Bhagavad Mahapuraan), once the Halahala poison started spreading everywhere, all Devas, along with Bhagavan Vishnu approached Bhagavan Shiva.

Bhagavan Neelkanth drinking poison (and Devi Parvati stops it in his throat)

तदुग्रवेगं दिशि दिश्युपर्यधो विसर्पदुत्सर्पदसह्यमप्रति ।
भीता: प्रजा दुद्रुवुरङ्ग सेश्वरा अरक्ष्यमाणा: शरणं सदाशिवम् | (Srimad Bhagavatam 8.7.19)

All Prajapatis, prayed to Bhagavan Shiva to save them from the poison.

देवदेव महादेव भूतात्मन् भूतभावन ।
त्राहि न: शरणापन्नांस्त्रैलोक्यदहनाद् विषात् ॥ (Srimad Bhagavatam 8.7.21)

Listening to their prayers Bhagavan Shiva told to Parvati that it is his duty to protect those who have taken shelter unto him and hence, he drank the poison. He also stated that by doing this Bhagavan Hari will be pleased. If Hari is pleased, then Bhagavan Shiva himself is also pleased.

श्रीशिव उवाच
अहो बत भवान्येतत् प्रजानां पश्य वैशसम् ।
क्षीरोदमथनोद्भ‍ूतात् कालकूटादुपस्थितम् ॥ (Srimad Bhagavatam 8.7.37)

आसां प्राणपरीप्सूनां विधेयमभयं हि मे ।
एतावान्हि प्रभोरर्थो यद् दीनपरिपालनम् ॥ (Srimad Bhagavatam 8.7.38)

पुंस: कृपयतो भद्रे सर्वात्मा प्रीयते हरि: ।
प्रीते हरौ भगवति प्रीयेऽहं सचराचर: ।
तस्मादिदं गरं भुञ्जे प्रजानां स्वस्तिरस्तु मे ॥ (Srimad Bhagavatam 8.7.40)

Devi Parvati also agreed with Bhagavan Shiva and nodded her approval. Having said thus, Bhagavan Shiva took the poison in his palms and drank.

तत: करतलीकृत्य व्यापि हालाहलं विषम् ।
अभक्षयन्महादेव: कृपया भूतभावन: ॥ (Srimad Bhagavatam 8.7.42)

Short answer to your question, though there were many Devatas present, they all prayed to Bhagavan Shiva to save them and hence, he drank the poison.

Om Namah Shivaaya!!!

Sarvam Sri Krishnaarpanam!!!